Table of contents

1. Introduction
   
2. Parent material and environment
   
3. Regional Distribution
   
4. Definition
   
5. Genesis
   
6. Characteristics of Ferralsols
   a. Morphological characteristics
   b. Physical characteristics
   c. Chemical characteristics
   
7. Management and Use of Ferralsols
   

Parent material:

• Strongly weathered material on old, stable geomorphic surfaces. Ferralsols are more prevalent in weathering material from basic rock than in acid silicious material.
  

Environment:

• Ferralsols occur in a wide variety of landscapes. Large contiguous areas of Ferralsols are found on old geomorphic surfaces, such as the Congo Basin and the Amazon Shield. These surfaces are peneplained and are comprised of mid-to end-Tertiary deposits, some of them pre-weathered. Ferralsols have developed on such deposits. In general these old peneplains are flat but some have been dissected. They are less common on younger, easily weathering rocks. In SE Asia and Oceania, where the landscapes are Quaternary, Ferralsols are formed on basic or ultrabasic rocks which are highly weatherable (see Beinroth et al., 1996; World Soil Resources Report 94, 2001).
  
• Some ferralsol landscapes are shown here:
  

  Fig.2 Ferralsol landscape on stable plateau in the semi-arid SW of Puerto Rico

  ( Source: Beinroth et al., 1996.)

  Fig.3 Puerto Rico: Ferralsols occupy the flat landforms

  ( Source: Beinroth et al., 1996.)

  Fig.4 Puerto Rico: Ferralsols occupy the flat landforms. Alisols on the slopes

  ( Source: Beinroth et al., 1996.)

  Fig.5 Ferralsols also occur in a dissected landscapes. Here under rainforest in NE Puerto Rico.

  ( Source: Beinroth et al., 1996.)

  Fig.6 Some Ferralsols are shallow because they developed on weathering products of limestone. It is highly probable that the soil material is a mixture of direct weathering products and transported infilling of a microkarst landscape. Here: in Puerto Rico

  ( Source: Beinroth et al., 1996.)

  Fig.7 Ferralsols at high elevations (> 1000 m) in the tropics have a humus-rich surface horizon. There is more than 16 kg of organic carbon to a depth of 1 m over an area of 1 m2. These soils are better suited for low input agriculture than those of the lowland tropics which contain less organic matter. Here: humic Ferralsol from Rwanda. Bananas are intercropped with maize; peanuts are in the foreground.

  ( Source: Beinroth et al., 1996.)

Fig.8 World Soil Map (Ferralsol)

( Source: FAO, 2001.)

• The only map available for an assessment of the global extent and geographic distribution is the Soil map of the World (FAO-UNESCO, 1971-1976) which has been digitized by FAO.
  
• The worldwide extent of Ferralsols is estimated at some 750 Mio. ha, almost exclusively in the humid tropics on the continental shields of South America (Brazil) and Africa (Zaire, southern Central African republic, Angola, Guinea and eastern Madagascar.
  
• Outside of the continental shields, in SE Asia and the Pacific Islands, ferralsols are restricted to regions with easily weatherable basic parent material and a humid-moist climate.
  

Fig.9 Ferralsol Hawai [Bo1 and Bo2 = oxic (ferralic B) horizon]

( Source: www.colorado.edu/.../geog_1011_f02/ study2_02.html)

• Must have a ferralic B horizon between 25 to 200 cm depth.
  
• Lacking a  nitic horizon within 100 cm from the soil surface.
  
• Lacking an argic horizon that has 10 % or more water-dispersible clay within 30 cm of its upper boundary (unless contains more than 1.4 % C).
  
• Cation exchange capacity ( CEC) < 16 cmol_c/kg clay (1 M NH₄OAc solution buffered to pH 7), see ( exchange capacity) .
  
• Has less than 10 % weatherable minerals in the silt fraction.
  
• Has less than 5 % volume of the soil which has rock structure.
  
• Lack of clear horizon boundaries. The profile appears to be very uniform from the surface to the parent material.
  

• Ferralitization (Desilication) is hydrolysis in an advanced stage. If the soil temperature is high and percolation intense (humid climate) all weatherable primary minerals will ultimately dissolve and be removed from the soil mass. Less soluble compounds such as iron and aluminum oxides and hydroxides ( Sesquioxides) and coarse quartz grains remain behind ( process of ferralitization) .
  
• The process of ferralitization is furthered by the following conditions:
  

1. Low soil-pH and low concentrations of dissolved weathering products in the soil solution promote desilication and build-up of high levels of (residual) Fe and Al.
   
2. Geo-morphological stability over long period of time is essential. Ferralitization is a slow process (also in the humid tropics).
   
3. Basic parent material contains relatively much Fe and Al in easily weatherable minerals, and little silica. Ferralitization proceeds much slower in acidic material that contains more quartz.
   
4. The Si-content in the soil solution remains higher than in soils in basic material; this Si combines with Al to the 1:1 clay mineral kaolinite (kaolinitization), particularly where internal drainage is impeded (see fig.5 below).

   Fig.10 Gibbsite and kaolinite formation under various drainage conditions

   ( Source: FAO, 2001.)

   
   

Morphological characteristics

• Ferralsols show a range of colors and other special features. Those in cold highland areas generally have an organic rich surface horizon.

  Fig.11 humic Ferralsol from Rwanda

  ( Source: ISRIC, Nl.)

  
  The lowland Ferralsols have a thin, light colored, organic surface horizon.
  
• Many Ferralsols have ( stone-lines) with the stones being quartz or petroplinthite gravel. Petroplinthite is also referred to as hard laterite.

  Fig.12 plinthic Ferralsols with stoneline, Thailand

  ( Source: ISRIC, NL.)

  
  
• The color of the Ferralsol is generally a function of the iron content in the original material or rock. The color is also related to the kind of Fe minerals with goethite producing yellow colors

  Fig.13 xanthic Ferralsols, Rwanda

  ( Source: ISRIC, NL.)

  and hematite, red colors (see fig.1).
  

• Ferrihydrite [Fe (OH)₃] is a common weathering product of Fe-rich parent material.  Hematite (Fe₂O₃) develops from ferrihydrite and gives mayn tropical soils their bright red color  rubefaction if:
  

1. Fe-concentration is high
   
2.  SOM is low (Fe-Humus-complexes inactivate Fe)
   
3. Temperature is high (accelerates de-hydration of Ferrihydrite and decomposition of SOM)
   
4. pH > 4.0 (otherwise Fe (OH)²⁺ Monomeres are formed)
   

• Goethite (FeOOH) develops when one or more of the conditions are not reached.  Goethite has a yellow-orange color (dominance of poorly crystallized Fe-hydroxides).

  Fig.14 Soil profile containing goethite

  Fig.15 “Yellowisation process”, i.e. the formation of goethite

  ( Source: www.soilmaps.it/soilphotos/ page_02.htm)

  
  

Physical characteristics

• The texture may vary from a sandy loam to a clay. Many Ferralsols have stable micro-aggregates which explain the excellent porosity and good permeability and favorable infiltration rates. The stable soil structure is based on the bonding between negatively charged LAC (kaolinite) and positively charged Fe/Al-oxides and is sometimes termed pseudo-sand-structure. Despite high clay contents, finger exercise (=a ped is gently pressed between the thumb and fore-finger) gives a loamy-sandy texture.
  

• Ferralsols with low contents of Fe and/or organic matter (e.g. xanthic Ferralsols in Surinam and Brazil) have less stable structure elements, especially the sandy ones. Surface sealing and compaction become serious limitations if such soils are taken into cultivation.
  

Chemical characteristics

• Ferralsols are chemically poor soils, and generally have low nutrient contents and low CEC due to the prevalence of variable-charged kaolinites and Fe/Al-oxides ( exchange capacity) .
  

Fig.16 Nutrient concentrations of a xanthic Ferralsol, Zaire

( Source: Bridges, 1997.)

• In deeper subsoil, nitrate may accumulate due to anion-sorption by positively charged Fe/Al-oxides (see chapter "Anion exchange capacity" in ( exchange capacity) .)
  
• Protonation of hydroxylic groups at low pH-values may boost the soil’s AEC to the extent that the AEC equals the CEC, and is termed ( point of zero net charge) .
  
• This can be detected by determining the  delta pH of the soil.
  

Management and Use

• Most ferralsols have good physical properties but the chemical fertility is poor to very poor.
  
• Under natural vegetation the nutrient cycling is very tight between the above-ground vegetation and the surface soil layer. The bulk of all cycling plant nutrients is contained in the plant biomass. Nutrient elements that are taken up by the roots are eventually returned to the surface soil with falling leaves and other plant debris. If this tight nutrient cycling is interrupted, e.g. after forest clearing for agricultural production, the root zone of the surface soil layer will rapidly become depleted of plant nutrients.
  
• As the natural nutrient retention capacity of kaolinites and sesquioxides is very low, soil organic matter (SOM) plays a crucial role to sustain CEC, N and P supply through SOM mineralization, to improve the water holding capacity, and counteract the AEC of Fe/Al-oxides (P-fixation). Hence, the management of ferralsols is the management of SOM.
  
• Management practices like mulching and manuring, the introduction of perennial crops (e.g. plantations), cover crops and enrichment fallows (=N fixing trees are incorprated in fallow period) are important practices to maintain SOM and soil fertility, and prevent surface soil erosion. Otherwise the productivity and fertility of the soils declines rapidly

  Fig.17 Productivity decline of soils under shifting cultivation

  ( Source: www.env.go.jp/en/w-paper/ 1990/eae190000000000.htm)

  
  
• Management practices are:
  

  Fig.18 Black pepper plantation; Brazil

  ( Source: ISRIC, NL.)

  Fig.19 Mixed cropping, E-Java, Indonesien

  ( Source: ISRIC, NL.)

  Fig.20 Plantain and banana plantation are another common land use on Ferralsols

  ( Source: ISRIC, NL.)

  Fig.21 Maize was planted as inter-row crop with coffee, Brazil. After harvest, the stubble has been plowed, leaving the soil surface susceptible to erosion.

  ( Source: Beinroth et al., 1996.)

  Fig.22 Rubber is a commonly planted estate crop on Ferralsols.

  ( Source: ISRIC, NL.)

  Fig.23 Brazil: forest clearing and establishment of pasture sites

  Fig.24 Gully erosion from the savanna region near Brasilia, Brazil as a result of land clearing and establishment of pasture

  ( Source: Beinroth et al., 1996.)

  Fig.25 Iguassu Falls at the border of Argentina and Brazil: the water is red due to very high sediment loading from land clearing activities in Brazil’s north

  ( Source: Beinroth et al., 1996.)

• Without maintaining adequate fallow periods for soil nutrient restoration (after 2-4 years of cropping) and sustaining soil organic matter and nutrient in the system, the fertility and productivity of ferralsols declines rapidly.
  
• Strong retention of phosphorous (P-fixation) is a problem of many Ferralsols.

  Fig.26 Phosporus response of maize crops planted on Ferralsols, Hawaii

  ( Source: Beinroth et al., 1996.)

  
  

• P was applied in small amounts and periodically to counteract leaching losses and fixation. In the absence of P, as in the foreground, the maize does not grow (see fig.20).
  
• The ferralic B horizon reacts with P-Ions:
  

1. Chemically bind them on Fe₇Al surfaces of the clay, i.e. positively charged oxides react with negatively charged PO₄^3- ions ( exchange capacity) .
   
2. May form independent Fe (FePO₄) or Al (AlPO₄) precipitates.